A fuel cell system is increasingly being used as a power source in a wide variety of applications. Fuel cell systems have been proposed for use in power consumers such as vehicles as a replacement for internal combustion engines, for example. Such a system is disclosed in commonly owned U.S. patent application Ser. No. 10/418,536, hereby incorporated herein by reference in its entirety. Fuel cells may also be used as stationary electric power plants in buildings and residences, as portable power in video cameras, computers, and the like. Typically, the fuel cells generate electricity used to charge batteries or to provide power for an electric motor.
Fuel cells are electrochemical devices which directly combine a fuel such as hydrogen and an oxidant such as oxygen to produce electricity. The oxygen is typically supplied by an air stream. The hydrogen and oxygen combine to result in the formation of water.
The basic process employed by a fuel cell is efficient, substantially pollution-free, quiet, free from moving parts (other than an air compressor, cooling fans, pumps and actuators), and may be constructed to yield only heat and water as by-products. The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells depending upon the context in which it is used. The plurality of cells is typically bundled together and arranged to form a stack with the plurality of cells commonly arranged in electrical series. Since single fuel cells can be assembled into stacks of varying sizes, systems can be designed to produce a desired energy output level providing flexibility of design for different applications.
Different fuel cell types can be provided such as phosphoric acid, alkaline, molten carbonate, solid oxide, and proton exchange membrane (PEM), for example. The basic components of a PEM-type fuel cell are two electrodes separated by a polymer membrane electrolyte. Each electrode is coated on one side with a thin catalyst layer. The electrodes, catalyst, and membrane together form a membrane electrode assembly (MEA).
In a typical PEM-type fuel cell, the MEA is sandwiched between “anode” and “cathode” gas diffusion media (GDM). The GDM and MEA are pressed between a pair of electronically conductive plates. The plates conduct current between adjacent cells internally of the stack in the case of bipolar plates and conduct current externally of the stack (in the case of monopolar plates at the end of the stack).
Electrical shorting through the MEA of a fuel cell is one of the major failure modes of the fuel cell. MEA shorting not only degrades the performance of the fuel cells, but also leads to membrane thinning and pinhole formation, which are undesirable. In the extreme case, the shorting can also lead to significant overheating, which not only can damage soft goods of the fuel cell, but also fuel cell plate hardware. Accordingly, it is desirable to devise a mitigation method to reduce the propensity of MEA shorting.
Despite the above-mentioned consequences from the MEA shorting, the fundamental mechanism of MEA shorting is poorly understood. Currently, there are two main theories in respect of shorting mechanisms. One is the “poking” mechanism of individual carbon fibers, and the other is the “indentation” mechanism of lumped fibers/binders (hard spots) in the GDM.
The “poking” of individual fibers through the GDM creates direct contact and passage for electrical conduction between the anode and the cathode sides of the MEA. Although this theory appears plausible, shorting tests have indicated that this type of mechanism is not the dominant mechanism causing MEA shorting.
Studies have shown that hard spots in the GDM are more likely the cause of MEA shorting. It has been a continuing challenge to provide an efficient and cost efficient method of minimizing hard spots in the GDM.
It would be desirable to produce a fuel cell stack assembly process to maximize MEA shorting resistance in the fuel cell to maximize a durability of the fuel cell stack, wherein soft goods and hardware used in the fuel cell are substantially unchanged, and a cost of the process is minimized.